Anatomy of Neck Muscles

Neck Muscles

• The thyrohyoid muscle is innervated by C1 fibers via the hypoglossal nerve (CN XII).

Scalene Muscles

• The scalene muscles have three parts: anterior, medius/middle, and posterior.
• They originate from the transverse processes of the 3rd to 6th cervical vertebrae.
• They insert into the 1st and 2nd ribs.
• They are innervated by C4, 5, and 6 (anterior rami of cervical nerves).
• When acting unilaterally, they elevate the 1st rib/2nd rib and laterally flex and rotate the cervical part of the vertebral column to the same side.
• When acting bilaterally, they cause cervical flexion.

Sternocleidomastoid Muscle

• The sternocleidomastoid muscle originates from the manubrium sterni and the medial 1/3 of the clavicle.
• It inserts into the mastoid process of the temporal bone and the occipital bone.
• It is innervated by the spinal part of the accessory nerve (CN 11) and C2 and 3.
• When contracting bilaterally, it causes cervical flexion and head extension.
• When contracting unilaterally, it causes ipsilateral flexion and contralateral rotation.

Suprahyoid and Infrahyoid Muscles

• The suprahyoid muscles include the digastrics, mylohyoid, stylohyoid, and geniohyoid (DMSG).
• The infrahyoid muscles include the thyrohyoid, omohyoid, sternothyroid, and sternohyoid (TOSS).
• The sternothyroid muscle is responsible for depressing the hyoid bone and larynx.
• The digastrics muscle elevates the hyoid bone and depresses the mandible.
• The mylohyoid muscle elevates the hyoid bone and floor of the mouth during swallowing.

Rectus Capitis Lateralis Muscle

• The rectus capitis lateralis muscle originates from the transverse process of the atlas (C1).
• It inserts into the jugular process of the occipital bone.
• It is innervated by the anterior rami of the 1st and 2nd cervical nerves (C1-C2).
• When acting bilaterally, it flexes the head on the neck.
• When acting unilaterally, it laterally flexes the head.

Rectus Capitis Anterior Muscle

• The rectus capitis anterior muscle originates from the lateral mass and transverse process of the atlas (C1).
• It inserts into the basilar part of the occipital bone.
• It is innervated by the anterior rami of the 1st and 2nd cervical nerves (C1-C2).
• It flexes the head.

Longus Capitis Muscle

• The longus capitis muscle originates from the anterior tubercles of the transverse processes of C3-C6.
• It inserts into the basilar part of the occipital bone.
• It is innervated by the anterior rami of the 1st to 3rd cervical nerves (C1-C3).
• It causes head flexion bilaterally and ipsilateral head rotation unilaterally.

Longus Colli Muscle

• The longus colli muscle has three segments: superior oblique, vertical portion, and inferior oblique.
• The superior oblique originates from the anterior tubercles of the transverse processes of C3-C5 vertebrae.
• The vertical portion originates from the anterior surfaces of the bodies of the lower three cervical and superior three thoracic vertebrae (C5-T3).
• The inferior oblique originates from the posterior fibers of the temporalis.

Mastigation

• The lateral pterygoid muscle pulls the neck of the mandible forward along with the articular disc.
• The temporalis, masseter, and medial pterygoid muscles are involved in the elevation of the mandible.
• The lateral pterygoid muscle is involved in the protrusion of the mandible, drawing the lower teeth forward over the upper teeth.
• The posterior fibers of the temporalis muscle are involved in the retraction of the mandible.

Teeth Development

• Deciduous teeth begin to erupt about 6 months after birth.
• All deciduous teeth have erupted by the end of 2 years.
• The lower jaw teeth usually appear first.
• There are 4 incisors, 2 canines, and 4 molars in each jaw.
• Teeth begin to erupt at 6 years of age, and the last tooth, the 3rd molar, typically erupts between 17 and 30 years of age.

Tongue

• The tongue is covered in a mucous membrane.
• The tongue is divided into right and left halves by a median fibrous septum.
• The upper surface of the tongue is divided into anterior and posterior parts by a V-shaped sulcus called the sulcus terminalis.
• The frenulum of the tongue is a fold of mucous membrane that connects the undersurface of the tongue to the floor of the mouth.
• The intrinsic muscles of the tongue are confined to the tongue and are not attached to bone.